Communication system and a method for controlling a connection in the communication system

ABSTRACT

A communication system in which terminal equipments capable of both voice and data service are interconnectable through a full-duplex connection or leg rate-adapted in compliance with CCITT V.110 or in the like manner and arranged to be attached V.24 interfaces, and to a method for controlling the connection. To enable the use of terminal equipments for both voice and data service in conjunction with such a rate-adapted leg, adapters provided at both ends of the leg utilize V.24 status signal states in the leg for hand-shaking end-to-end signalling events between the terminal equipment.

BACKGROUND OF THE INVENTION

The invention relates to a communication system in which interconnectionbetween terminal equipments capable for both voice and data serviceconsist of at least one connection leg, which is rate-adapted incompliance with the CCITT recommendation V.110 or in a like manner andattached to V.24 interfaces, and to a method for controlling such aconnection.

In a communication system of this type, the terminal equipments maycomprise a data terminal adapted for a V.110 rate-adapted full-duplexconnection by a terminal adapter, and a telephone handset which is, forinstance, connected to a network termination or integrated in the dataterminal for telephone conversation.

Examples of terminal equipments suitable for both data and voice servicemodes are the Telefax Group 3 terminal equipment and a personal computertelecommunication software which are adapted by a terminal adapter e.g.for an ISDN V.110 rate-adapted 64 kbit/s UDI bearer service or a GSMPLMN 9.6/4.8/2.4 kbit/s UDI bearer service. The terminal equipment maybe full-duplex or half-duplex, and it comprises a standard ornon-standard session protocol corresponding to the OSI layers 4 to 7 forthe management and use of the voice/data service modes. The networktermination equipment may thus be NT in ISDN or MT2 in GSM PLMN, forinstance.

The terminal equipments of the Telefax Group 3 have not previouslycomprised a V.24 interface; instead, they have always utilized a modeminterface and a 2-wire connection (modem connection through a telephonenetwork, for instance). As the same transmission connection has therebybeen able to transmit both voice and baseband modem signalling, theterminal equipments have been able to perform the transition from dataservice mode to voice service mode, for instance, in compliance with theapplied management protocol of the services of the OSI layers 4 to 7(e.g. CCITT T.30 or T.4) without any need to indicate the change of theservice mode to the used transmission network or without any need forthe transmission network to react to the change of service mode bysomehow altering the configuration of the transmission connection.

Communication systems utilizing V.110 connections have the drawback thatalthough the terminal equipments comprise a handset for telephoneservice, the V.110 rate-adapted transmission path is not able totransfer voice. However, the use of telephone service during a call is anormal function in the Telefax Group 3, for instance. Similarly, whenreceiving an incoming data call by a personal computer, it is necessaryto answer the call by speech to enable the terminal user to start anappropriate application.

ISDN and GSM PLMN subscriber signallings will contain an In-CallModification procedure, by which the speech connection can, if required,be changed into a V.110 rate-adapted data connection, and vice versa, anunlimited number of times by replacing the PCM coding with the UDIcoding, and vice versa.

However, it has not been specified how the communication network couldreact to a service mode change performed within the OSI layers 4 to 7between terminal equipments designed for modem connections, and as aresult of it take the necessary steps to adapt the network for the newservice mode e.g. by starting the In-Call Modification procedure tochange a V.110 rate-adapted leg in the signal path into a speechconnection.

Generally speaking, the problem is that an application utilizing a V.110leg, such as a telefax service, and associated adapting means signalbetween each other functions which are significant for the call control(change of service mode, change of transmission rate, release of call).However, the call control is assigned to the network termination e.g. inthe mobile telephone of the GSM system.

SUMMARY OF THE INVENTION

The object of the invention is to allow full utilization of integratedvoice/data terminal equipments in communication systems deployingrate-adapted connection segments attached to V.24 interfaces.

The invention is based on the principle that the terminal equipmentfunctions related to the OSI layers 4 to 7 are adapted by a terminaladapters or alternatively by terminal adapter and network adapterrespectively in the terminal end and the network end for a V.24 statussignal state to be transmitted within a frame over the rate adaptedconnection leg.

As already mentioned above, the transmission properties of the GSMsystem require reconfiguration of the traffic channel when the servicemode (voice-data) or the transmission rate is changed, wherefore the GSMsystem comprises a terminal adapter and a network adapter which arespecific for each service and which monitor the application signallingbetween the terminal equipments in order to be able to initiate theprocedures required for the reconfiguration of the traffic channel.

In the invention, application parts forming part of the service areallowed to perform a hand-shaking of transactions significant for thecall by means of V.24 status signal states from one end of theconnection to the other. Such a hand-shaking procedure establishes asignalling connection between the above-mentioned adapters and enablesprocedures preventing the failure of the service due to theinappropriate mode of the rate-adapted connection segment. In the GSMsystem, for instance, there is thus three independent signallingprotocols in effect: (1) GSM call control in compliance with the GSMstandard 04.08 in a signalling channel Dm, (2) application protocolbetween the terminal equipments, such as the telefax applicationprotocol T.30/T.4, and (3) hand-shaking between a terminal adapter and anetwork adapter associated with the service by means of V.25 statussignal states.

Since the rate adaptation of the V.24 signal states to the V.110 framesis effected by the network termination, the network termination caneasily also monitor the V.24 status signal states and initiate the callcontrol procedures as required by the specified hand-shaking protocole.g. on the signalling channel of the GSM system. One advantage of thisarrangement is that the terminal adapter associated with the telefaxservice, for instance, can be realized independently by anothermanufacturer in such a way that the communication of the terminaladapter with the network termination is based on the fixed V.24interface protocol instead of an equipment or software interfacespecific for the mobile telephone manufacturer.

The invention also allows the connection management to be automated asextensively as possible so that the reliability of the connectionmanagement functions can be increased (e.g. by ensuring that theconnection shall always be released also from the direction of thesubscriber equipment) and the use is facilitated in such a way that theuser need not perform any additional procedures deviating from thenormal operation of the terminal equipment for the configuration of theconnection in cases where the connection comprises a rate adaptedsegment.

The network terminal may effect call control signalling, channelconfiguration and some other automatic adaptation in response to atleast one predetermined V.24 status signal state.

The invention allows the attachment of a voice/data terminal equipmentto a V.110 connection or other similar connection without that thesubscriber signalling of the communication network needs to comprise anyother special functions besides the normal In-Call modificationprocedure.

The invention increases the number of functions of the terminal adapterand the network adapter; however, it has the advantages of offeringbetter support to the service functions and being based on standardsignalling.

In the preferred embodiment of the invention connection adaptation meansalways carry out a hand-shaking procedure with each other by means ofthe V.24 status signal states through the rate-adapted leg after initialsynchronization of the connection in conjunction with the set-up of therate-adapted leg. As a result of this hand-shaking procedure, apredetermined first V.24 status signal state is provided for theconnection segment, which status signal state is maintained unchangedthroughout the data transmission in the data service mode.

The connection adaptation means monitor the communication between theterminal equipments, and on detecting the transmission of a disconnectmessage, they carry out a second hand-shaking procedure with each otherby means of the V.24 status signal states through the leg, as a resultof which a second predetermined V.24 status signal state causing theconnection to be cleared is provided onto the leg.

Preferably, the connection adaptation means further monitor thecommunication between the terminal equipments, and on detecting thetransmission of a request to change service mode, they carry out a thirdhand-shaking procedure with each other by means of the V.24 statussignal states through the leg, as a result of which a thirdpredetermined V.24 status signal state starting the In-Call Modificationprocedure is provided onto the leg. The leg is restored from the voicemode to the data mode by the In-Call Modification procedure started bymanual intervention on the network termination, which is followed by thesynchronization of the leg and said first hand-shaking procedure, as aresult of which the leg reassumes said first V.24 status signal state.

Preferably the connection adaptation means further monitor thecommunication between the terminal equipments, and on detecting arequest to change transmission rate, they perform a hand-shakingprocedure by means of the V.24 status signal states, as a result ofwhich the network adapter initiates a Channel-mode-modify procedurecomplying with the GSM standard 04.08 or the like procedure and theconnection is caused to assume a fourth predetermined mode as anindication of the pending change of transmission rate. After theChannel-mode-modify procedure has been successfully completed in thenetwork termination, the connection adaptation means perform a newhand-shaking procedure by means of which the V.24 status signal statesare restored to the first predetermined mode so as to indicate that bothterminal equipments are ready to continue the communication at the newchannel rate. This procedure can speed up the change of transmissionrate, which may be of importance in certain applications (e.g. telefaxservice).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by means ofexemplifying embodiments with reference to the attached drawings, inwhich:

FIGS. 1A and 1B show communication systems according to the invention,in which terminal equipments are interconnected through a V.110connection alone, and through a V.110 connection and a transit network,respectively; and

FIGS. 2 to 8 show signalling diagrams illustrating the starting of aterminal session, the terminating of the terminal session, and thechange of the service mode and transmission rate of a V.110 connectionin a GSM PLMN system (digital mobile radio system).

DETAILED DESCRIPTION

As shown in FIGS. 1A and 1B, the terminal equipments in thecommunication system according to the invention may comprise a dataterminal 1 adapted to a V.24 interface by a terminal adapter 2, andfurther adapted for a V.110 rate-adapted full-duplex connection bynetwork termination 3, and a telephone handset 4 connected to thenetwork termination 3 or integrated in a data terminal 1 for telephoneconversation. The V.110 connection described herein is originally adigital transmission channel developed for ISDN (Integrated ServicesData Network) technology. It can be attached to a V.24 interface and itoffers a possibility of transferring V.24 status signal states. Theinvention can, however, also be applied to other transmission channelswhich are similar to the V.110 channel and capable of transferring V.24status signal states. An example of such transmission channels is theCCITT recommendation V.120, which is being developed.

The CCITT recommendation for a V.110 rate-adapted transmission channelis disclosed in CCITT Blue Book:V.110.

The CCITT recommendation for a V.24 interface is disclosed in CCITT BlueBook:V.24. As used here, a V.24 status signal state refers to acombination of the states of individual status signal (control) signalsin the V.24 interface, such as CT105, CT106, CT107, CT108.2 and CT109. Amultiple of such V.24 status signal states can be used for handshakingprocedures.

Without restricting the general scope of the invention, only one exampleaccording to the invention of the use of the above-mentioned statussignal states will be described. It is obvious to one skilled in the artthat the object of the invention can also be realized in some other wayby using the above-mentioned or some other status signals of the V.24interface in such a way that the adaptation means in the opposite endsof the rate adapted leg are able to monitor the mode of each other andmaintain the coordination necessary for the service. Furthermore, someof the status signals can be reserved for a local hand-shaking procedurein such a way, for instance, that a predetermined status signal state isset after the synchronization of the V.110 connection between thenetwork termination and the terminal adapter. Such a local hand-shakingprocedure limits the possibilities for the hand-shaking of status signalstates over the network as some of the status signal states are not ableto propagate transparently between the adaptation means.

It is also obvious to one skilled in the art that the hand-shakingprocedures to be described in the text below can be modified extensivelywithout deviating from the principle of the invention. In any case, thehand-shaking procedures have to be designed in such way that they areunambiguous, and a signalling mode forming part of one hand-shakingprocedure must not inappropriately result in the initiation of anotherprocedure. For instance, the connection releasing procedure has to bespecified in such a way that it cannot result in status signal stateswhich might be interpreted as a request for the change of service modeby the network terminal.

As shown in FIG. 1B, the terminal equipments may be interconnectedsolely through a V.110 rate-adapted network, that is, e.g. ISDN or GSMPLMN.

Alternatively, as shown in FIG. 1A, the terminal equipments may beinterconnected through a transit network 7 (PSTN) in addition to theV.110 rate-adapted connection or a leg, in which case a network adapter6 is provided between the V.110 connection and the transit network.There are several possibilities for transit networks, e.g.:

from GSM PLMN through PSTN to GSM PLMN with a modem as the networkadapter 6,

from GSM PLMN to the terminal equipment of PSTN with a modem as thenetwork adapter,

from ISDN through PSTN to ISDN with a modem as the network adapter 6,

from ISDN to the terminal equipment of PSTN with a modem as the networkadapter 6,

from GSM PLMN through ISDN to GSM PLMN with mere synchronization anduser data rate-adapting as the network adapter 6.

Terminal equipments suitable for both data and voice service modeinclude the Telefax Group 3 terminal equipment or the personal computertelecommunication software designed for V.24 interface, which is adaptedby the network termination for an ISDN V.110 rate-adapted 64 kbit/s UDIbearer service or a GSM PLMN 9.6/4.8/2.4 kbit/s UDI bearer service. Theterminal equipment may be full-duplex or half-duplex, and it comprises astandard or non-standard terminal session protocol corresponding to theOSI layers 4 to 7 for the management and use of the voice/data services.The network termination equipment is thus e.g. NT in ISDN or MT2 in GSMPLMN. Examples of such service procedures within the OSI layers 4 to 7are the CCITT recommendations T.4 and T.30, which can be found in CCITTBlue Book:T.4 and CCITT Blue Book:T.30.

In the following the invention will be described by way of example bymeans of a communication in which the hand-shaking of the Telefax Group3 terminal equipments in compliance with the CCITT recommendation T.30is adapted for GSM PLMN by using PSTN as a transit network 7 and amodem-based network adapter 6.

The subsystems of the GSM PLMN telefax service comprise a mobile radiounit (e.g. telephone) and a network adapter for the PSTN, as shown inFIG. 2. The mobile unit may comprise a telefax terminal (Fax-TE), atelefax terminal adapter (Fax-TA) and a network termination (TAF/MT2).The telefax terminal may be a conventional PSTN terminal equipment. Thenetwork termination may contain radio functions, signalling functions,mobility functions, and an adapter TAF offering a V.24 interface. Thetelefax terminal adapter may be an adapter complying with GSM 03.45. Thenetwork adapter for PSTN may comprise a transmission rate adapter (RA)and a telefax network adapter (Fax-IWU). Fax-TA and Fax-IWU arehalf-duplex in a direction towards a 2-wire connection and full-duplextowards GSM PLMN. The call control of the transparent telefax servicesof GSM PLMN is specified in the GSM recommendation 03.45. The terms usedherein for the equipments and functions also mainly comply with the GSMstandard.

In the GSM recommendation 03.45, the call control of the transparenttelefax services of GSM PLMN specifies the adaptation of a half-duplextelefax service for a full-duplex PLMN connection in an inexact mannerand fails to recite how the telefax terminal adaptation means andtelefax<network adaptation means may need to coordinate their concertedaction. This may lead to failures because of false assumptions of stateand capability of the adaptation means connected to the other end of therate adapted connection segment.

This is a recent problem because the terminal equipments of the TelefaxGroup 3 did not previously comprise a V.24 interface but they havealways utilized a 2-wire interface.

The invention provides a solution to the above-mentioned problems byintroducing the concept of hand shaking to allow terminal adaptationmeans and network adaptation means to remain adequately informed of theoperational status of the other end.

By means of the invention, a conventional telefax terminal or GSMtelefax device can be attached to a GSM PLMN connection without that thesubscriber signalling (complying with GSM 04.08) needs to comprise anyspecial telefax service functions.

The invention also allows the change of the service mode into telephoneservice and vice versa, which is an essential function of telefaxservice, in such a way that the functions of the telefax terminal areadapted for the normal In-Call Modification procedure of GSM PLMN.

The invention increases the number of functions of Fax-TA and Fax-IWU,but a major advantage is a better support for the features of theservice functions.

The invention complements the standard call control of the transparenttelefax services of ETSI/GSM at its different stages in the followingway.

Starting a Terminal Session

The starting of a terminal session according to the invention isdescribed in the following with reference to FIG. 2.

At first a connection is set up between the terminal equipments and inconjunction therewith the V.110 connection of GSM PLMN is synchronizedin full compliance with the standard specified by ETSI/GSM.

After synchronization at least some of the V.24 status signals aretransmitted transparently between the terminal adapter and the networkadapter. In the following, it is assumed without restricting the generalscope of the invention that the status signals CT106, CT107 and CT109are transmitted from the network adapter to the terminal adapter.Correspondingly, it is assumed that the status signals CT105 and CT108.2are transmitted from the terminal adapter to the network adapter.

A Mobile Unit Sets Up a Connection

After synchronization the network adapter Fax-IWU indicates that it hastransmitted a calling tone (CNG) and received a called tone (CED) bytransmitting the status signals CT109 OFFtoON and CT106 OFFtoON to theterminal adapter Fax-TA through the V.110 connection in a V.110 frame.

The terminal adapter Fax-TA acknowledges the status signals CT109 andCT106 by setting CT105 OFFtoON. When the state CT108.2 of the terminalequipment is additionally ON, a predetermined V.24 status signal stateis provided onto the connection, which status signal state indicates tothe terminal adapter and the network adapter that the terminal sessionhas started and the terminal adapter Fax-TA and the network adapterFax-IWU may initiate the hand-shaking of the carrier as specified in theGSM standard 03.45.

The terminal adapter and the network adapter keep the V.24 status signalstate unchanged throughout the terminal session unless applicationsignalling between the terminal equipments calls for performing otherprocedures significant for the rate adapted leg. To detect suchapplication signals, the terminal adapter Fax-TA and the network adapterFax-IWU monitor the communication between the terminal equipment incompliance with the GSM standard 03.45.

A PSTN Data Terminal Sets Up a Connection

After synchronization, the network adapter Fax-IWU indicates that it hasreceived a calling tone (CNG) and initiated the transmission of a calledtone (CED) by transmitting the status signals CT109 OFFtoON and CT106OFFtoON to the terminal adapter Fax-TA through the V.110 connection in aV.110 frame.

The terminal adapter Fax-TA acknowledges the status signals CT109 andCT106 by setting CT105 OFFtoON. When the state CT108.2 of the terminalequipment is additionally ON, a predetermined V.24 status signal stateis provided onto the connection, which status signal state indicates tothe terminal adapter and the network adapter that the terminal sessionhas started and the terminal adapter Fax-TA and the network adapterFax-IWU may initiate the hand-shaking of the carrier as specified in theGSM standard 03.45.

The terminal adapter and the network adapter keep the V.24 status signalstate unchanged through the terminal session unless applicationsignalling between the terminal equipments calls for performing otherprocedures significant for the rate-adapted leg. To detect suchapplication signals, the terminal adapter Fax-TA and the network adapterFax-IWU monitor the communication between the terminal equipment incompliance with the GSM standard 03.45.

Call Clearing Procedure

In the following the clearing of a terminal session and the releasing ofa connection will be described with reference to FIGS. 3 and 4. Theterminal adapter Fax-TA and the network adapter Fax-IWU monitor theterminal session, and on detecting a disconnect request, they establisha predetermined V.24 status signal state, which may lead to the networktermination to take action to automatically clear the call.

The Mobile Unit Ends the Session

FIG. 3 shows how the mobile unit (Fax-TE) ends up the terminal session.The telefax terminal Fax-TE ends the terminal session by sending andisconnect message T.30:DCN, which is detected and forwarded by theterminal adapter Fax-TA monitoring the communication. The networkadapter Fax-IWU also detects and forwards the disconnect message. FAX-TAtransmits the V.24 status signals CT105 ONtoOFF and CT108.2 ONtoOFF.After having completed the transmission towards PSTN, Fax-IWUacknowledges by transmitting the V.24 status signals CT106 ONtoOFF,CT107 ONtoOFF and CT109 ONtoOFF. Both adapters assume an idle state. Inresponse to such determined V.24 status signal state, the networktermination TAF/MT2 is thereby able to generate the clearing of the callin a normal way if the terminal user has not initiated the clearing ofthe call from the user interface.

The PSTN Terminal Ends the Session

FIG. 4 shows how a terminal (not shown) connected to a public telephonenetwork (PSTN) ends the terminal session (generally a case in which thedisconnection message comes from the direction of the transmissionnetwork). The PSTN terminal ends the terminal session by transmittingthe disconnection message T.30:DCN. The network adapter Fax-IWU detectsand forwards the disconnection message and, and after disconnection,transmits the V.24 status signals CT106 ONtoOFF, CT107 ONtoOFF and CT109ONtoOFF. The terminal adapter Fax-TA also detects the disconnectionmessage, and acknowledges by the V.24 status signals CT105 ONtoOFF andCT108.2 ONtoOFF. The terminal adapter FAX-TA and the network adapterFax-IWU assume an idle state. In response to such determined V.24 statussigna state, the network termination TAF-MT2 is thereby able to generatethe clearing of the call in a normal way if the terminal user has notinitiated the clearing of the call from the user interface by an MMIcommand.

Change of the Type of Service Mode During a Call

FIGS. 5 and 6 show the change of the type of service mode during aterminal session. T.30 specifies an optional Voice Request function, bymeans of which one of the terminal users is able to request the other tochange over to telephone conversation and then return to the dataservice mode during the session by a hand-shaking procedure between theterminal equipments. The terminal adapter Fax-TA and the network adapterFax-IWU monitor the terminal session, and on detecting a change of thetype of service mode, they generate a V.24 status signal state for thePLMN connection in such a way that, if required, the network terminationTAF is able to start the In-Call Modification procedure in a standardmanner on the basis of this V.24 status signal state.

The Mobile Unit Requests for Change of Service

FIG. 5 illustrates the change of service requested by the mobile unit(Fax-TE). Fax-TE requests for the change of service (Voice Request) bytransmitting PRI-Q in a normal way. The terminal users have 10±5 secondstime to initiate a speech connection. The terminal adapter Fax-TAmonitors the hand-shaking procedure between the terminal equipments, andon detecting a service mode changing message, it forwards it. Thenetwork adapter Fax-IWU also detects the service mode changing messageand after having forwarded it, it transmits the V.24 status signalsCT107 ONtoOFF and CT109 ONtoOFF. The terminal adapter Fax-TA therebygenerates the V.24 status signal CT105 ONtoOFF for the V.110 connection,and the network adapter Fax-IWU acknowledges by the V.24 status signalCT106 ONtoOFF, and both of them assume an idle state to wait for therestoration of the data mode. In addition, on detecting a service modechanging message, the terminal adapter Fax-TA keeps the V.24 statussignal CT108.2 in the ON state, which is the opposite of the case whenit detects the disconnection message. Without restricting the generalscope of the invention, it is assumed in this example that the networktermination TAF/MT2 reacts to the V.24 status signal state so generatedby initiating a normal In-Call Modification procedure, if the terminaluser has not already initiated it by the user interface. The In-CallModification procedure also causes the network adapter Fax-IWU to bedisconnected from the line.

When the terminal users want to return to the terminal session, one ofthem generates the In-Call Modification procedure by the user interface.The In-Call Modification procedure starts the synchronization of theV.110 connection and the reconnection of the network adapter Fax-IWU tothe line. The network adapter Fax-IWU returns to the terminal sessionand acknowledges by means of CT107 OFFtoON and CT109 OFFtoON. Theterminal adapter Fax-TA also returns to the terminal session and changesCT106 OFFtoON, which is acknowledged by the network adapter with CT106OFFtoON.

The terminal adapter Fax-TA and the network adapter Fax-IWU now bothknow that they are both in the data state and ready for hand-shaking thecarrier.

The PSTN Equipment Requests for Change of Service Mode

FIG. 6 illustrates a request for change of service made by a terminalequipment (not shown) connected to a public telephone network PSTN(generally a case in which the request for the change of service modecomes from the direction of the transmission network). When the PSTNterminal equipment requests for the change of service mode, thehand-shaking procedure is different for the network terminal and theterminal adapter, whereas it is similar as above for the V.24 statussignal state.

In FIG. 6, a PSTN subscriber starts the Voice Request function bytransmitting PIN/PIP in a normal way as an acknowledgement of anMPS/EOM/EOP message transmitted by MS. MS answers by transmitting aPRI-Q message. The terminal users have 10±5 seconds time to initiate aspeech connection.

If MS is fully integrated, the MMI command of the Voice Request maydirectly start the In-Call Modification procedure.

If Fax-TA is a separate device, it detects PIN/PIP and PRI-Q transmittedas an acknowledgement of it. Fax-TA thereby keeps the V.24 status signalCT108.2 in the state ON and transmits the V.24 status signal CT105ONtoOFF. Fax-IWU thereby assumes the Voice Request mode and acknowledgesby the V.24 status signals CT106 ONtoOFF, CT107 ONtoOFF and CT109ONtoOFF.

Without restricting the general scope of the invention, it is assumed inthis example that the network termination TAF reacts to the V.24 statussignal state so generated by initiating a normal In-Call Modificationprocedure. The In-Call Modification causes the IWU/Fax-IWU to bedisconnected from the line.

Return to the telefax mode takes place similarly as in the case of FIG.5.

Change of the Transmission Rate

FIGS. 7 and 8 illustrate one possible function according to theinvention in connection with the change of the transmission rate. InFIG. 7, the telefax terminal (not shown) on the PSTN side initiates thechange of the page transmission rate. In FIG. 8, the change of thetransmission rate is initiated by the telefax terminal equipment Fax-TEof the mobile telephone.

The terminal equipments possibly negotiate the transmission rate to beused with each other and possibly change it during the session e.g. inorder to improve a detected inferior quality of the connection. Intelefax services, for instance, the terminal equipments may negotiateand renegotiate the page transmission rate. The change of thetransmission rate in such a way requires reconfiguration of the leg, forwhich purpose a Channel-mode-modify procedure is specified in GSM PLMN.

In GSM PLMN, transmission rate changes are performed solely by themobile telephone exchange, and so the network adapter Fax-IWU controlsthe function. To detect the rate changing messages of the terminalequipments, the network adapter Fax-IWU monitors the communicationbetween the terminal equipments.

On detecting a transmission rate changing message between the terminalequipments, the network adapter Fax-IWU sets the V.24 status signalCT105 ONtoOFF. The terminal adapter Fax-TA acknowledges by setting CT105ONtoOFF. Thus a predetermined V.24 status signal state has beenestablished indicating that both the terminal adapter FAX-TA and thenetwork adapter FAX-IWU are prepared for the imminent change in thetransmission rate.

The network adapter Fax-IWU starts the change of the rate of the channelthrough the signalling of the call control in compliance with the GSMstandard 04.08. When the network termination TAF/MT2 receives a rateadaptation message from the call control of the mobile unit MS, itconfigures the V.110 rate adaptations and effects the terminal adapterto restore the V.24 status signal state to normal:CT105 OFFtoON. Suchfeedback from the network termination to the terminal adapter can bebased on local V.24 status handshaking as mentioned above. The networkadapter Fax-IWU detects CT105 status signal changes and acknowledges bychanging CT107 OFFtoON. This indicates that both adapters have completedthe reconfiguration of the V.110 connection and the channel can again beutilized by the terminal equipments. When the hand-shaking takes placeby means of the V.24 status signal states between the adapters, thechannel can be returned to use more rapidly than when waiting for thecompletion of the Channel-mode-modify procedure through the callcontrol. This is important in synchronous applications, as such telefaxservice, in which there is no flow control available.

As a summary, it can be stated that the following features in particularare characteristic of the invention when applied to GSM PLMN:

Fax-TA and Fax-IWU generate a full-duplex V.24 status signal state forthe PLMN connection at the connection set-up stage after thesynchronization of the PLMN connection,

Fax-TA and Fax-IWU generate a full-duplex V.24 status signal state forthe PLMN connection after the Fax session in such a way that, ifrequired, TAF is able to generate a connection release,

Fax-TA and Fax-IWU generate a full-duplex V.24 status signal state forthe PLMN connection after the Voice Request started during the sessionin such a way that Fax-TA and Fax-IWU assume the Voice Request mode towait for the return to the telefax mode, and that, if required, TAF isable to generate the In-Call Modification procedure,

Fax-TA and Fax-IWU generate a full-duplex V.24 status signal state forthe PLMN connection after the In-Call Modification procedure in such away that Fax-TA and Fax-IWU return after the Voice Request mode to thetelefax mode as specified in CCITT:T.30,

Fax-IWU and Fax-TA perform a hand-shaking procedure by means of apredetermined V.24 status signal state so as to indicate a transmissionrate change to be made,

Fax-TA and Fax-IWU perform a hand-shaking procedure after thereconfiguration of the transmission rate and subsequent synchronizationof the V.110 connection by means of a normal full-duplex V.24 statussignal state.

For the V.110 connection in general, the features particularlycharacteristic of the invention can be summarized as follows:

the terminal adapter and the network adapter generate a full-duplex V.24status signal state for the V.110 connection at the connection set-upstage after the synchronization of the connection,

the terminal adapter and the network adapter generate a predeterminedfull-duplex V.24 status signal state for the V.110 connection inresponse to the disconnect message,

when the terminals interrupt the terminal session and change over to thevoice service mode, the terminal adapter and the network adaptergenerate a full-duplex V.24 status signal state for the V.110 connectionin such a way that the terminal adapter and the network adapter assume astand-by mode to wait for the return to the terminal session, and that,if required, the terminal adapter is able to generate the In-CallModification procedure,

the terminal adapter and the network adapter generate a full-duplex V.24status signal state for the V.110 connection after the In-CallModification procedure in such a way that the terminal adapter and thenetwork adapter return from the stand-by mode to the terminal session.

The description and the drawings are only intended to illustrate thepresent invention. In their details, the method and the datatransmission system according to the invention may vary within the scopeof the accompanying claims.

I claim:
 1. A method for controlling a connection between terminalequipments in a communication system in which the connection includes atleast one leg attached to V.24 interfaces and rate-adapted in compliancewith the CCITT recommendation V.110 or an equivalentcomprising:utilizing V.24 status signal states in said at least one legfor hand-shaking end-to-end signalling events between the terminalequipments which are significant to said at least one leg.
 2. The methodaccording to claim 1, further including:determining the V.24 statussignal states of said leg by utilizing said end-to-end signallingevents.
 3. The method according to claim 1, wherein:said end-to-endsignalling events between the terminal equipments significant to the legcorrespond to procedures of at least one of OSI layers 4 to
 7. 4. Amethod according to claim 1, comprising:monitoring messages transmittedby said terminal equipments in accordance with OSI layers 4 to 7; andgenerating a corresponding one of said V.24 status signal states forsaid at least one leg when a message significant to said at least oneleg is detected as a result of said monitoring.
 5. The method accordingto claim 1, comprising:providing adapters at both ends of said at leastone leg for adapting terminal equipment signals for said at least oneleg; and operating said adapters to always carry out a firsthand-shaking procedure by means of said V.24 status signal states afteran initial synchronization of said at least one leg, said firsthand-shaking procedure generating a first predetermined one of said V.24status signal states for said leg, which first status signal state ismaintained unchanged throughout a data mode of the connection.
 6. Themethod according to claim 5, comprising:restoring said at least one legfrom a voice mode to a data mode by an In-Call-Modification procedurestarted by one of said terminal equipments, and thereafterresynchronizing said at least one leg and generating said one of saidV.24 status signal states for said at least one leg utilizing said firsthand-shaking procedure.
 7. The method according to claim 1,comprising:providing adapters at both ends of said at least one leg foradapting terminal equipment signals for said at least one leg; andoperating said adapter to monitor the communication between saidterminal equipments, and when said adapters detect transmission of adisconnect message, operating said adapters to carry out a hand-shakingprocedure utilizing said V.24 status signal states, said hand-shakingprocedure generating a predetermined one of said V.24 status signalstates for said at least one leg, which status signal state results in arelease of said connection.
 8. The method according to claim 1,comprising:providing adapters at both ends of said at least one leg foradapting terminal equipment signals for said at least one leg; operatingsaid adapters to monitor the communication between the terminalequipments; and when said adapters detect a request for changing servicemode, said adapters carrying out a hand-shaking procedure utilizing saidV.24 status signal states, said hand-shaking procedure generating apredetermined one of said V.24 status signal states for said at leastone leg, which status signal state results in a standardIn-Call-Modification procedure.
 9. The method according to claim 1,comprising:providing adapters at both ends of said at least one leg foradapting terminal equipment signals for said at least one leg; operatingsaid adapters to monitor communication between said terminal equipments;and upon thereby detecting a message indicating a change in transmissionrate between said terminal equipments, causing said terminal equipmentsto carry out a hand-shaking procedure with each other utilizing saidV.24 status signal states, said hand-shaking procedure generating apredetermined one of said V.24 status signal states for said at leastone leg, which status signal state results in carrying out a standardChannel-mode-modify procedure.
 10. The method according to claim 9,comprising:after changing of the transmission rate of said at least oneleg initiated by said V.24 status signal state has been completed, saidadapters carrying out another hand-shaking procedure utilizing said V.24status signal states, said other hand-shaking procedure generatinganother one of said V.24 status signal states, which other status signalstate causes said at least one leg be returned to useability by saidterminal equipments.
 11. A communication system capable of establishinga connection between terminal equipments, comprising:at least one leghaving two ends attached to V.24 interfaces and rate-adapter incompliance with the CCITT recommendation V.110 or an equivalent;adaptation means provided at both ends of said leg for adapting terminalequipment signals for said at least one leg, said connection adaptationmeans being structured for utilizing V.24 status signal states in saidat least one leg for hand-shaking end-to-end signalling events betweensaid terminal equipments which are significant to said at least one leg.12. The system according to claim 11, wherein:said adaptation means arestructured to monitor communication between said terminal equipments fordetecting said end-to-end signalling events between said terminalequipments significant to said at least one leg and to utilize said V.24status signal states for hand-shaking at least one of said end-to-endsignalling events.
 13. The communication system according to claim 12,wherein:said adaptation means comprise means for carrying out a firstpredetermined hand-shaking procedure by means of said V.24 status signalstates for generating a first one of said V.24 status signal states forsaid at least one leg in response to an initial synchronization of saidat least one leg.
 14. The communication system according to claim 13,wherein:said adaptation means are structured to carry out, in responseto detecting of a request to change service mode in the communicationbetween the terminal equipments, a second predetermined hand-shakingprocedure by means of said V.24 status signal states for generating asecond one of said V.24 status signal states for said at least one leg.15. A system according to claim 14, wherein:said adaptation means arestructure to carry out a third predetermined hand-shaking procedure bymeans of said V.24 status signal states for generating a third one ofsaid V.24 status signal states for said at least one leg in response todetection of a disconnection message in the communication between saidterminal equipments.
 16. The communication system according to claim 13,wherein:said adaptation means are structured to carry out in a voicemode said hand-shaking procedure by means of said V.24 status signalstates for generating said first one of said V.24 status signal statesfor said at least one leg in response to an In-Call-Modificationprocedure started by one of said terminal equipments.
 17. Thecommunication system according to 12, wherein:one of said adaptationmeans comprises a terminal adapter means; and wherein a networktermination means is associated with said terminal adapter means forproviding rate-adaptation and signalling functions; a connection beingset up between said terminal equipments via said terminal adapter meansand network termination means such that said at least one leg israte-adapted by said network termination means; said network terminationmeans being structured for monitoring said V.24 status signal states insaid at least one leg and effecting call control signalling, channelconfiguration or other automatic adaptation in response to at least oneof said V.24 status signal states.
 18. The communication systemaccording to claim 17 wherein:said network termination means beingstructured to effect, in response to the detection of said second statussignal state in said rate-adapted leg, a call control signalling for anIn-Call-Modification procedure.
 19. A communication system according toclaim 17, wherein:said network termination means being structured toeffect, in response to the detection of said third status signal statein said rate-adapted leg, a call control signalling for a call clearingprocedure.
 20. A communication system according to claim 11,wherein:said terminal equipments are telefax devices structured forexchanging therebetween messages complying with CCITT T.4 or T.30 andaccording to at least one of OSI layers 4, 5, 6 and 7.